COMP: Fix build with system-installed expat 2.0.1

In cmXMLParser::ReportXmlParseError we were accidentally passing a value
of type 'XML_Parser*' to expat methods instead of 'XML_Parser'.  It was
not caught because XML_Parser was just 'void*' in the cmexpat version.
Newer system-installed expat versions catch the error because XML_Parser
is now a pointer to a real type.  This correct the type.

Source/cmXMLParser.cxx

@@ -228,7 +228,7 @@ void cmXMLParserCharacterDataHandler(void* parser, const char* data,
 //----------------------------------------------------------------------------
 void cmXMLParser::ReportXmlParseError()
 {
-  XML_Parser* parser = static_cast<XML_Parser*>(this->Parser);
+  XML_Parser parser = static_cast<XML_Parser>(this->Parser);
   this->ReportError(XML_GetCurrentLineNumber(parser),
                     XML_GetCurrentColumnNumber(parser),
                     XML_ErrorString(XML_GetErrorCode(parser)));